True, when the metre was defined by a particular metal bar in a French safe (or before that a pendulum swing/fraction of the Earth's radius).

But for nearly thirty years it's been defined in terms of the speed of light, so a "light year" is arguably very much part of the same system (the "year" in question being a defined number of SI seconds).

P.S. Even in countries which still use Imperial measurements, their official definition is in terms of the SI (metric) units. The only things you can really say are a different system are the weird astronomical ones like parsecs.

True, when the metre was defined by a particular metal bar in a French safe (or before that a pendulum swing/fraction of the Earth's radius).

But for nearly thirty years it's been defined in terms of the speed of light, so a "light year" is arguably very much part of the same system (the "year" in question being a defined number of SI seconds).

P.S. Even in countries which still use Imperial measurements, their official definition is in terms of the SI (metric) units. The only things you can really say are a different system are the weird astronomical ones like parsecs.